Co--Cr--Mo alloys designated as ASTM F75, "STELLITE" Alloy No. 21, "CARPENTER CCM" and GADS "VITALLIUM" are used in medical devices, particularly surgical implants. Such devices require good wear resistance for their articulating surfaces and high strength to handle the loads imposed during use. The "STELLITE" Alloy No. 21 has been extensively used in implant devices. In the late 1980's, the upper weight percent limit for nickel in ASTM F75 (F75-87) was lowered from 3% to 1% in order to reduce the risk of in vivo allergic reactions to nickel. The lower nickel specification of F75-87 separated the two alloy grades and currently both "STELLITE" Alloy No. 21, containing up to 3% nickel, and ASTM F75-87, containing up to 1% nickel, are used in cast form for medical implant devices.
Cast material suffers from microstructural defects such as porosity and carbide segregation (carbide networking or carbide pooling). Porosity leads to localized corrosion and degradation of mechanical properties of a cast implant. Carbide segregation in a cast alloy is believed to be a cause of corrosion related failure of cast implants. It is known to subject a cast implant to a homogenization heat treatment in order to reduce carbide segregation. Hot isostatic pressing (HIP) of cast parts is used to eliminate porosity resulting from the casting operation.
The purpose of the post-casting thermal treatments is to increase the performance of the implants in use by eliminating local defects in the microstructure. However, such thermal treatments reduce the overall hardness and strength of the material and, thus, adversely affect the wear resistance provided by the alloys. To offset the loss of hardness and strength resulting from a post-casting thermal treatment, an aging treatment can be used to increase the strength of a cast surgical implant that has been homogenized or HIP'd.
Because of the difficulties encountered with cast material, the use of cast and wrought material was pursued. Manufacturing of the F75-87 alloy by standard cast/wrought practices has proven to be difficult because the carbide segregation present in the as-cast ingot makes the material difficult to hot work. In an effort to make the F75-87 alloy easier to hot work without resorting to special techniques, the base F75-87 alloy composition was modified by lowering the carbon content to a nominal 0.05% and adding up to 0.20% nitrogen. The composition of the modified alloy is defined in ASTM F799-87 (F799). The "CARPENTER CCM" alloy is typical of such grades. The reduction in carbon reduces the amount of carbide segregation present in the cast ingot. The addition of nitrogen increases the strength of the alloy and compensates for the loss of strength that would otherwise result from the lower carbon content. The nitrogen-strengthened material is frequently prepared by thermomechanical processing (TMP), a combination of thermal treatment and simultaneous mechanical hot working below the recrystallization temperature, to achieve the higher mechanical properties and hardness specified in ASTM F799 for surgical implants. Typically, TMP is performed at temperatures in the range of 1700-1900F.
TMP requires close control to assure that all of the material possesses the same amount of mechanical working to achieve uniform and reproducible mechanical properties from bar to bar or part to part. This is particularly true during the forging of finished or near-finished products from bar-stock, where several reheats and forging passes are employed to shape the product. The forging practice changes with the product geometry, particularly its thickness. The ability to achieve ASTM F799 properties becomes difficult or impossible depending on the ability to forge the material at the proper temperature to achieve the desired amount of residual stress in the forged part. This means using TMP'd bar-stock that meets ASTM F799 mechanical properties does not guarantee a forger that the forged product he makes will meet the properties specified in ASTM F799. Part-to-part reproducibility is also a concern as is die wear, which increases as lower temperatures are used to "work in" the desired mechanical properties.
Nitrogen, in the form of nitrides, does not retard grain growth as well as carbon in the form of carbides. That characteristic presents a problem in connection with porous coated, i.e., uncemented, implants. Such implants were developed to provide better fixation in the body than cemented implants. To produce a porous coated implant, metal powder or fine wire is sintered onto the alloy substrate. The sintering operation requires heating the article to a high temperature to properly bond the powder to the substrate. Unless special precautions are taken, such a high temperature cycle can cause grain growth, segregation, and microporosity in the substrate material with a resulting loss of corrosion resistance and strength, particularly fatigue strength.
To remedy the grain size problem, a dispersion strengthened version of the "VITALLIUM" alloy (GADS "VITALLIUM") has been developed. As described in U.S. Pat. No. 4,668,290, the GADS "VITALLIUM" alloy contains a dispersion of metal oxides that offset the adverse effects of the sintering process. The GADS "VITALLIUM" alloy can only be produced using powder metallurgy processing as opposed to cast/wrought processing.
The GADS "VITALLIUM" alloy contains a dispersion of relatively hard metal oxide particles which appreciably strengthens the alloy, but can also render it relatively difficult to process. The GADS "VITALLIUM" alloy powder is consolidated and hot worked to produce bar-stock. The finished bar-stock of the alloy may contain small ultrasonic indications resulting from microporosity that forms during hot working of the alloy. Such indications apparently result from the hot working parameters selected (billet temperature or amount of reduction per pass) and/or the difference in plasticity between the matrix material and the dispersed metal oxides. Bar-stock of the GADS "VITALLIUM" alloy has been found to be difficult to cold straighten and the material is known to break on occasion during the cold-straightening process, apparently because of stress intensification resulting from the presence of the metal oxide particles. The limited ability to cold straighten bar forms of the GADS "VITALLIUM" alloy, makes it more difficult and expensive to process the material into finished bar.
The GADS "VITALLIUM" alloy is also difficult to machine because of the dispersed metal oxides. Consequently, this alloy is supplied primarily in forge-bar form. The forge bar can be further hot worked to eliminate any microporosity.
The use of nitrogen as a strengthening agent for porous coated implants is known, as described in T. Kilner et al., Nitrogen Strengthening of F75-76 Alloys, Proceedings of the 11th Annual Meeting of the Society for Biometals, Apr. 25-28, 1985. That reference shows that the yield strength of porous coated F75-76 alloy (3.00% max. Ni) can be increased to about 75 ksi by nitriding the alloy. However, the increased yield strength does not meet the 120 ksi minimum yield strength required by ASTM F799. The non-nitrided material has a carbon content of 0.07% and provides a yield strength of about 50 ksi (340 MPa).
Combinations of nitrogen and carbon have also been studied in connection with the ASTM F75 alloy to improve the alloy's ductility as well as its yield and tensile strengths. U.S. Pat. No. 3,865,585 describes an alloy containing up to 0.5% carbon and 0.15 to 0.5% nitrogen. Despite the improvement in ductility reported for that alloy, its yield and tensile strengths do not meet the mechanical requirements of ASTM F799.
Powder metallurgy processing has also been used to produce surgical implants from the "STELLITE 21" and F75-87 alloys. It is generally known that powder metallurgy products possess a more homogeneous microstructure than cast/wrought products. The mechanical properties of the known powder metallurgy products generally meet the requirements of ASTM F75-87 (65 ksi yield strength min.), but do not meet those of ASTM F799 (120 ksi yield strength min.). The known powder metallurgy products can be thermomechanically processed in order to meet the mechanical properties of ASTM F799. The powder metallurgy products can also be subjected to an aging heat treatment to increase the hardness and to improve the mechanical properties such as yield strength. Annealing a powder metallurgy product results in a significant reduction in the strength and hardness of the alloy, such that the powder metallurgy alloy product does not have a hardness of at least 35 HRC, the minimum hardness specified in ASTM F799. A hardness equal to or greater than 35 HRC is desired in the implant industry to achieve the best performance of implant devices during in vivo use.